1 Stacks (Continued) and Queues Array Stack Implementation Linked Stack Implementation The java.util.Stack class Queue Abstract Data Type (ADT) Queue ADT Interface Queue Design Considerations The java.util.Queue interface Reading L&C 3 rd : 4.4, 4.6, nd :

2 Array Stack Implementation We can use an array of elements as a stack The top is the index of the next available element in the array top Object of type T null T [ ] stack

3 Array Stack Implementation push – O(n) public void push (T element) { if (size() == stack.length) expandCapacity(); stack [top++] = element; } worst case: O(n) How about the average case: almost constant time.

4 Array Stack Implementation pop() – O(1) public T pop() throws EmptyStackException { if (isEmpty()) throw new EmptyStackException(); T result = stack[--top]; stack[top] = null; // removes “stale” reference return result; } The “stale” reference stored in stack[top] would prevent garbage collection on the object when the caller sets the returned reference value to null – ties up resources

5 Linked Stack Implementation We can use the same LinearNode class that we used for LinkedSet implementation We change the attribute name to “top” to have a meaning consistent with a stack Object of type T topLinearNode next; T element; Object of type T LinearNode next; T element; null count Object of type T

6 Linked Stack Implementation push – O(1) public void push (T element) { LinearNode temp = new LinearNode (element); temp.setNext(top); top = temp; count++; } Note the difference between the stack push method and the LinkedSet add method –There is no check for already present or not

7 Linked Stack Implementation pop – O(1) public T pop () throws EmptyStackException { if (isEmpty()) throw new EmptyStackException(); T result = top.getElement(); top = top.getNext(); count--; return result; } Note the difference between the stack pop method and the LinkedSet remove method –There is no specified target as a parameter

8 The java.util.Stack Class The java.util.Stack class is part of the Java collections API. It is a subclass of the java.util.Vector class which is a subclass of java.util.AbstractList java.util.Stack inherits some methods from its superclasses that are inappropriate for a stack collection – a bad design decision! A good programmer will avoid using those methods on a Stack object

9 The java.util.Stack Class java.util.Stack has a search method that returns the distance from the top of the stack to the first occurrence of that object on the stack, e.g. search returns 1 for the top element of the stack If the object is not found, search returns -1 The search method is O(n) Does this method violate the principles of a stack as a data structure?

10 Queue Abstract Data Type A queue is a linear collection where the elements are added to one end and removed from the other end The processing is first in, first out (FIFO) The first element put on the queue is the first element removed from the queue Think of a line of people waiting to be served at RMV.

11 A Conceptual View of a Queue Rear of Queue (or Tail) Adding an Element Removing an Element Front of Queue (or Head)

12 Queue Terminology We enqueue an element on a queue to add one We dequeue an element off a queue to remove one We can also examine the first element without removing it We can determine if a queue is empty or not and how many elements it contains (its size) The L&C QueueADT interface supports the above operations and some typical class operations such as toString()

13 Queue ADT Interface > QueueADT + enqueue(element : T) : void + dequeue () : T + first() : T + isEmpty () : bool + size() : int + toString() : String

14 Queue Design Considerations Although a queue can be empty, there is no concept for it being full. An implementation must be designed to manage storage space For first and dequeue operation on an empty queue, this implementation will throw an exception Other implementations could return a value null that is equivalent to “nothing to return”

15 Queue Design Considerations No iterator method is provided That would be inconsistent with restricting access to the first element of the queue If we need an iterator or other mechanism to access the elements in the middle or at the end of the collection, then a queue is not the appropriate data structure to use

16 The java.util.Queue Interface The java.util.Queue interface is in the Java Collections API (extends Collection) However, it is only an interface and you must use an implementing class LinkedList is the most commonly used implementing class For a queue of type objects: Queue myQueue = new LinkedList ();

17 The java.util.Queue Interface The names of the methods are different Enqueue is done using: boolean offer(T element) // returns false if full Dequeue is done using either: T poll() // returns null value if empty T remove() // throws an exception if empty Peek is done using either: T peek() // returns null value if empty T element() // throws an exception if empty